archlinuxpkgs: add explicit_only option and outdated packages support

[Xartoks]

Implements the ideas from #217

• Adds an explicit_only config option (default: true) to filter out dependency packages
• Adds outdated package detection for both official repos and AUR, respecting explicit_only
• Adds action to update outdated packages

[abenz1267]

wouldn't one want a periodic check for the outdated packages?

[Xartoks]

Yes, I thought about it. The same problem exists when installing/uninstalling packages — the cache becomes stale. I'm not sure how to handle this other than a periodic refresh every X minutes, which feels a bit overkill, but I'm not sure.